1. Field of the Invention
The present invention relates to medical devices and, more specifically without limitation, to implanted medical devices.
2. Description of the Related Art
Epilepsy affects about 2.3 million Americans, and its direct and indirect annual costs amount to approximately $12.5 billion. Although anti-epileptic drugs are useful, 20-30% of persons are not helped by them and up to 30% of those treated have intolerable or serious side effects.
Recently published studies demonstrate the importance of quantitative analysis of brain signals for automated warning and blockage of seizures, optimization of existing therapies and development of new ones. Cardiac activity is under cerebral control. That is, certain changes in global, regional or focal brain activity, either physiological or pathological, modify heart activity. Epileptic seizures are one of the pathological brain states associated with changes in heart activity including but not limited to changes in heart rate, most frequently an increase and referred to as ictal tachycardia, or in other indices of cardiac function such as R-R variability. The incidence of heart changes increases as the seizure spreads outside its site of origin to other brain regions being, for example invariably present in all subjects with primarily or secondarily generalized tonic-clonic seizures (“convulsions”), in whom purportedly, most or all of the brain is involved. These changes reflect shifts in the ongoing interplay between sympathetic and parasympathetic influences, which can be quantified using time or frequency domain methods of analysis. For example, tachycardia precedes electrographic onset of temporal lobe seizures by several seconds, as ascertained via scalp electrodes (EEG), while combined activation of parasympathetic and sympathetic systems as estimated by using spectral analysis of oscillations in R-R intervals at respiratory and non-respiratory frequencies, may be detectable minutes in advance of seizure onset. Since these changes may precede visible electrographic or behavioral manifestations indicative of seizures and even of the so-called “aura,” they may have predictive value. Real-time prediction or detection of epileptic seizures, based on extracerebral sources such as the heart, is of great clinical and practical value as it obviates the reliance on cerebral signals which are highly complex and of high dimensionality and whose origin may not only be difficult to localize but quite often requires invasive intracranial implantation of electrodes or other sensors.
While methods presently exist to detect seizures using cardiac signals and quantify their characteristics, for example as described in U.S. Pat. No. 6,341,236 which is incorporated herein by reference in its entirety, no system for logging the times of seizures and their quantitative characteristics, such as date and time of occurrence, and duration based on the degree of cardiac changes, and for using this information in the objective assessment of seizure frequency and of therapeutic intervention, presently exists. This is partially due to the impact of artifacts (noise) on EKG signal analysis which can lead to inaccuracies in heart rate assessments.
Thus, the need exists for a system and method for logging seizures, or other events originating in the brain that impact cardiac activity, and associated event characteristics such as frequency, duration, intensity, and severity. Moreover, this system and method needs to be robust in the presence of artifacts or other sources of noise. The need also exists for a minimally invasive system and method to provide effective and objective means for assessing the efficacy of therapies used to control seizures.